The National Park Service says graffiti has been found at the Lincoln Memorial, the Washington Monument and the World War II Memorial.

U.S. Park Police spokeswoman Sgt. Anna Rose said Tuesday that the messages written in permanent marker were discovered over the holiday weekend. Similar graffiti was found on a power box along the National Mall.

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Rose says the message written on the Washington Monument references President John F. Kennedy's assassination and the Sept. 11, 2001 attacks.

Park service spokesman Mike Litterst says because of similarities in what's written and the handwriting, it's thought the graffiti was left by the same person.

U.S. Park Police are investigating and a news conference was planned for 11 a.m. EST.